A step up-step down or a buck-boost switching regulator produces an output voltage VOUT that can be above, below, or at the same level as input voltage VIN. FIG. 1 shows a conventional step up-step down switching regulator including an inductor L, switches S1 and S2 coupled between the input VIN and the inductor L, and switches S3 and S4 coupled between the inductor L and the output VOUT. Input capacitor CIN and output capacitor COUT are respectively coupled to the input VIN and output VOUT. A sense resistor RSENSE is provided for sensing current. Current sense inputs SNS+ and SNS− arranged at both sides of the sense resistor RSENSE can supply input signals to a current sense comparator for determining the inductor current IL. A voltage divider composed of resistors R1 and R2 provides the output voltage VOUT to a control circuit 10 for regulation.
In particular, the control circuit 10 controls switches S1 to S4 to provide peak current mode regulation in a boost mode when VIN is lower than VOUT, and to provide valley current mode regulation in a buck mode when VIN is higher than VOUT. As illustrated in FIG. 2, in a buck mode (VIN>VOUT), switch S3 (SWITCH 3) is always turned off, and switch S2 (SWITCH 2) is controlled by an error amplifier in the control circuit 10. The error amplifier provides an error signal representing a difference between the output voltage VOUT and a reference voltage VREF. Also, FIG. 2 shows a clock signal CLOCK used to control switching of the switches S1-S4, and the current IL in the inductor L. In this mode, switch S4 is always turned on, and switch S1 is controlled to provides synchronous rectification.
As shown in FIG. 3, in a boost mode (VIN<VOUT), switch S2 (SWITCH 2) is always turned off and switch S3 (SWITCH 3) is controlled by the error amplifier. In this mode, switch S1 is always turned on, and switch S4 is controlled to provide synchronous rectification. FIG. 3 also shows the clock signal CLOCK and the inductor current IL.
When VIN is close to VOUT, the step up-step down regulator operates in a buck-boost mode, in which all switches are turned on and off each cycle. Two cases can exist in the buck-boost mode—the input voltage VIN is slightly less than the output voltage VOUT, or VIN is slightly higher than VOUT.
When the input voltage VIN is slightly less than the output voltage VOUT, switches S1 and S3 turn on at the start of the clock cycle CLOCK. If the error amplifier forces the switches off before some minimum on-time TMIN, then switches S2 and S4 will turn on for a minimum on-time. After switch S2 turns off, switch S1 will turn on for the remainder of the clock cycle, and switch S4 will remain in the on-state. FIG. 4 illustrates operation of switches S2 and S3 in this mode, and shows the clock signal CLOCK and the inductor current IL.
When the input voltage VIN is slightly higher than the output voltage VOUT, switches S2 and S4 turn on at the start of the clock cycle CLOCK. If the error amplifier forces the switches off before some minimum on-time TMIN, then switches 1 and 3 will turn on for the minimum on-time TMIN. After switch S3 turns off, switch S4 will turn on for the remainder of the clock cycle, and switch I will remain in the on-state. FIG. 5 illustrates operation of switches S2 and S3 in this mode, and shows the clock signal CLOCK and the inductor current IL.
This conventional switching scheme, works well when the regulator operates purely in a buck mode, or in a boost mode, but not so well in buck-boost mode. During buck-boost mode the switches S1-S4 do not always turn on at a fixed frequency, which may result in increased electromagnetic interference (EMI).
Hence, there is a need for a control circuit that would control switches of the step up-step down regulator so as to provide switching in a buck-boost mode at a fixed frequency.